Oncogenesis involves alterations both in protooncogenes and in tumor suppressor genes. Understanding tumorigenesis, particularly when diverse cell types or specific cell lineages are involved, will require the use of manipulable in vivo systems such as transgenic and "knock out" mouse models. We have established transgenic tumor models based on the fact that tumor virus oncoproteins such as SV4O T antigen (T-Ag) inactivate or alter key cellular regulatory proteins, including p53 and the pRB family proteins. By using tissue-specific promoters to express wild-type and mutant forms of T-Ag, we have explored the function of these tumor suppressor proteins within the animal. Recently, we determined that the specific impact of p53 inactivation in vivo can be different depending on the cell type. In thymocytes, the loss of p53-dependent apoptosis is required for initiation of tumorigenesis, an observation that is consistent with a role for p53 in DNA damage-induced cell cycle check point control. This cell type currently represents the best in vivo experimental system for exploring the molecular details of the "checkpoint model" for p53 loss in tumorigenesis and is the focus of the current proposal. In this proposal we will utilize the transgenic T cell tumorigenesis model as well as p53-null mice to test the following hypotheses: (1) That errors in the T cell receptor gene recombination process signal p53-dependent apoptosis. Existing mutant TAg transgenic mice will be bred to specific mice which are defective in the recombination process (RAG knock-out,TcR transgenic and scid mice) and the impact on tumorigenesis will be measured. (2) That informative genetic aberrations will be present in T cell tumors and will include alterations in specific loci affecting tumorigenesis. We will examine T-Ag-induced and p53-null thymomas for genetic aberrations using simple sequence length polymorphism (SSLP) and comparative genomic hybridization (CGH) analyses to detect differences in normal and tumor genomes. We wish to determine the types and frequency of genomic alterations accumulating in the absence of p53 function, and to detect specific changes that could affect T cell tumorigenesis. (3) That one or more cyclin-Cdk inhibitors are downstream effectors of p53-dependent apoptosis in thymocytes. We will analyze Cdk- associated proteins in irradiated and untreated thymocytes from transgenic mice expressing mutant forms of TAg, as well as p53-null mice, to obtain correlative differences. The hypothesis that the p53-regulated p2l protein (Cipl/Waf1), is a downstream mediator of the apoptotic response to irradiation will be tested in vivo.